If archaeologists know how pottery styles, glazes, and techniques have changed over time they can date sites based on the ratio of different kinds of pottery.
This also works with stone tools which are found abundantly at different sites and across long periods of time.
When this occurs bodily tissues can be preserved, but it as extremely rare occurrence. When an organic item is buried in sediment, heat and pressure causes the release of hydrogen and oxygen, which leaves behind a carbon residue.
Fossils can also form when an organic entity is encased in tar, resin, or even ice.
One half-life is the amount of time required for of the original atoms in a sample to decay.
Over the second half-life, of the atoms remaining decay, which leaves of the original quantity, and so on.
The earliest-known hominids in East Africa are often found in very specific stratigraphic contexts that have implications for their relative dating.
Wiens has a Ph D in Physics, with a minor in Geology.